Elsevier

Clinical Neurophysiology

Volume 116, Issue 11, November 2005, Pages 2648-2663
Clinical Neurophysiology

The composite N1 component to gaps in noise

https://doi.org/10.1016/j.clinph.2005.08.001Get rights and content

Abstract

Objective

To indicate whether the double peaked N1 to gaps in continuous white noise is a composite of onset and offset responses to transients or whether it reflects higher processing such as change or mismatch detection and to assess the role of attention in this process.

Methods

Evoked potentials were recorded to two binaural stimulus types: (1) gaps of different durations randomly distributed in continuous white noise; and (2) click pairs at intervals identical to those between gap onsets and offsets in the continuous noise stimulus. Potentials to these stimuli were recorded while subjects read a text and while detecting gaps in noise or click pairs.

Results

Potentials were detected to all click pairs and to gaps of 5 ms or longer, corresponding to the subjects' psychoacoustic gap detection threshold. With long gap durations of 200–800 ms, distinct potentials to gap onset and gap offset were observed. The waveforms to all click pairs and to offsets of long gaps were similar and single-peaked, while potentials to gaps of 10 ms and longer, and potentials to onsets of long gaps were double-peaked, consisting of two N1 negativities, 60 ms apart, irrespective of gap duration. The first (N1a), was more frontal in its distribution and similar to that of clicks. The second (N1b) peak's distribution was more central/temporal and its source locations and time course of activity were distinct. No effects of attention on any of the varieties and constituents of N1 were observed.

Conclusions

Comparing potentials to gap onsets, to click pairs and to gap offsets, suggests that potentials to gap onsets involve not only sound onset/offset responses (N1, N1a) but also the subsequent pre-attentive perception of the cessation of an ongoing sound (N1b). We propose that N1b is distinct from change or mismatch detection and is associated with termination of an ongoing continuous stimulus. We propose to call it the N(egation)-process.

Significance

A constituent of the N1 complex is shown to be associated with the pre-attentive perception of termination of an ongoing stimulus and to have distinct scalp distribution and intracranial sources.

Introduction

The temporal resolution of the auditory system is critical for speech perception and discrimination and for sound localization. Measuring auditory temporal resolution faces the problem of spectral changes that accompany changes in the time pattern of the sound. One way to overcome this limitation is to use interruptions in white noise, because the spectral content of white noise remains flat even when abrupt gaps are introduced. Thus, the threshold for detecting gaps in broadband noise has been used as a psychoacoustic measure of auditory temporal resolution. Gap detection threshold in comfortably loud broadband noise is typically 2–3 ms (Eddins and Green, 1995, Moore, 1997, Penner, 1977, Plomp, 1964, Zeng et al., 1999), increasing to 20 ms with noise levels near hearing threshold (Irwin et al., 1981, Zeng et al., 1999).

Gap detection is typically examined with bursts of white noise interrupted by gaps of silence. Such a complex stimulus includes gap onset (noise offset) followed by gap offset (noise onset), in addition to burst onset (e.g. Eggermont, 2000, Rupp et al., 2002). Recently, psychoacoustic and evoked potentials measures of auditory temporal processes were compared in normal-hearing individuals and in patients with auditory neuropathy using gaps in continuous noise (Michalewski et al., 2005). In normal subjects evoked potentials (N1/P2 components) were recorded in response to gaps as short as 5 ms in both active and passive conditions. Gap evoked potentials in the patients appeared only with prolonged gap durations (10–50 ms). There was a close association between gap detection thresholds measured psychoacoustically and electrophysiologically in both normals and in auditory neuropathy subjects. The study concluded that auditory evoked potentials to gaps in continuous noise can provide objective measures of auditory temporal processes.

The latter study (Michalewski et al., 2005) reported that when the cortical potentials to gaps were triggered from gap onset (offset of the noise), the N1 was broad compared to N1 to tone or noise onset. N1 consisted of two separate components in most subjects when gap durations were longer than 20 ms: an early component peaking at 90 ms, similar in latency to a stimulus onset N1 and a later component peaking at approximately 150 ms. At gap durations of 20 ms or shorter, the N1 consisted of a single component approximately centered between the earlier (90 ms) and the later (150 ms) components. The later of the two peaks was usually the larger and frontally prominent.

Two possible explanations to account for the double N1 peak to gap onset (noise offset) were proposed (Michalewski et al., 2005): One explanation attributed the later of the two peaks to a separate perceptual distinction made by the subjects to longer gap durations (Phillips, 1999). Thus, given its scalp distribution (frontal) and timing (approximately 150 ms) the second peak may be attributed to a mismatch negativity to stimulus change (Naatanen, 1992) or a negative component related to auditory change, an ‘acoustic change process’ (Jones and Perez, 2002). The other explanation included the possibility that the double peaked N1 may involve a combination of offset/onset responses, or the interaction of these responses in the averages. Neither of these alternative explanations could be ruled out.

The purpose of this study was to indicate whether the early ERP components to onset and to offset of gaps in continuous white noise are simple composites of the well known onset and offset responses to transients or whether they also reflect a more advanced process such as change detection. Therefore, in this study potentials to gap onset and offset were separated and compared with the better studied potentials to short transient stimuli (clicks). Potentials were recorded while subjects attended or ignored the stimuli, and compared to assess the effect of attention on these components.

Section snippets

Subjects

Thirteen, right handed, normal hearing subjects, 18–25 years old participated in the study. Subjects were paid for their participation and all procedures were approved by the institutional review board for experiments involving human subjects (Helsinki Committee).

Stimuli

Binaural stimuli were used throughout this study to avoid confounding the scalp distribution of evoked potentials by contralateral or ipsilateral stimulation. Thus, any lateralization of brain activity would be attributed to

Behavioral gap detection

Reaction times and performance accuracy of gap detection for the gap durations of this study are presented in Fig. 2. In general, accuracy levels for gaps of 10 ms and longer approached 90% on average, dropping to about 60% with gaps of 5 ms duration and further dropping to below chance with the shorter gaps. This effect of gap duration on performance accuracy was significant [F(5,55)=117.77, P<0.0001]. Reaction times were around 500 ms for gaps 10 ms and longer, approaching 600 ms for gaps of 5 ms

Discussion

In this study potentials to short gaps in noise and to onset and offset of long gaps in noise were compared with the better studied potentials to short transient stimuli (clicks), while subjects attended or ignored the stimuli. Many of the findings of this study corroborate the findings of an earlier study (Michalewski et al., 2005). Thus, no effects of attentional state on the N1–P2 complex were observed, and amplitudes and latencies of the constituents of this complex were the same, whether

Acknowledgements

Henry Michalewski and Arnold Starr introduced us to the intriguing complexity of evoked potentials to gaps in continuous noise. Discussions with Ilan laufer contributed to our appreciation of the processes associated with N1a and N1b. This study was partially supported by the Rappaport Family Institute for Research in the Medical Sciences.

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